1. Field of the Invention
This invention relates to a method of applying an annealing separator to grain oriented magnetic steel sheets electrostatically, to prevent their sticking together when they are annealed at a high temperature. More particularly, it is concerned with a method which enables the industrial application of an annealing separator to grain oriented magnetic steel sheets.
2. Description of the Prior Art
Grain oriented magnetic steel sheets are annealed at a high temperature of at least 900.degree. C. to achieve transformation to the (110)[001] orientation by secondary recrystallization after at least one cycle of cold rolling and annealing. An annealing separator is applied to the surfaces of the steel sheets to prevent their sticking together which may occur during their high temperature annealing.
Various refractory metal compounds have been proposed for use as the annealing separator. They include, for example, CaCO.sub.3 and BaCO.sub.3 (Japanese Pat. Nos. 179,337 and 185,395), Al.sub.2 O.sub.3, ZrO.sub.2, MgO and TiO.sub.2 (Japanese Pat. Publication No. 27533/1967), Al.sub.2 O.sub.3 and CaO (Japanese Pat. Publication No. 27531/1967) and MgO (Japanese Pat. Publications Nos. 12451/1976 and 31296/1977). For grain oriented magnetic steel sheets containing silicon, it is common to use an annealing separator consisting mainly of magnesium oxide, since it not only prevents the steel sheets from sticking together, but also forms a glass-like film on the steel surface during high temperature annealing by a solid-phase reaction with a sub-scale layer consisting mainly of SiO.sub.2. This glass-like film consists mainly of forsterite (Mg.sub.2 SiO.sub.4) formed by the solid-phase reaction between SiO.sub.2 in the sub-scale on the steel surface and MgO in the annealing separator. It is a useful backing for an insulation film, and improves its heat resistance and insulating property.
Various alloying elements are added to grain oriented magnetic steel as a normal grain growth inhibitor to inhibit the growth of primary recrystallization grains during the high temperature annealing which is carried out to effect transformation to the (110)[001] orientation by secondary recrystallization as hereinabove stated. Examples of the alloying elements include Mn, S, Al, N, V, B, Cu, Sn, Sb, Se and Mo. If a compound, such as MnS, AlN, VN, Cu.sub.2 S or MnSe, is precipitated, it inhibits the growth of normal grains by pinning the boundary migration of primary recrystallization grains. In the event the intergranular segregation of an element, such as B, Sn, Sb or Mo, takes place, it is dragged with the boundary migration of primary recrystallization grains and resists their migration to thereby inhibit the growth of normal grains. It is known that the transfer of mass on the steel interface contributes greatly to the precipitation or segregation of the alloying element, and therefore, the gas permeability and reactivity of the annealing separator have an important bearing on the secondary recrystallization of steel.
If the steel contains, for example, [S] as the inhibitor, it is necessary to remove it after secondary recrystallization in order to improve its magnetic characteristics. The annealing separator consisting mainly of MgO promotes the desulfurization of steel effectively by absorbing [S] in the vicinity of its surface and lowering its [S] potential.
Thus, the annealing separator consisting mainly of magnesia enables:
(1) prevention of sticking together of steel sheets during high temperature annealing; PA0 (2) formation of a glass-like film; PA0 (3) stabilization of secondary recrystallization; and PA0 (4) promotion of purification (mainly desulfurization) of steel. It is, therefore, a very useful annealing separator for grain oriented magnetic steel sheets. It is, however, evident that magnesia is not the only material for a useful annealing separator for grain oriented magnetic steel sheets, but that any other material can be used if it prevents the sticking together of steel sheets, and if it does not hinder the effective secondary recrystallization of steel. PA0 (1) A drying furnace is required to dry the slurry, and increases the costs of equipment and energy which are required for the production of grain oriented magnetic steel sheets. PA0 (2) The preliminary heating or soaking of steel sheets at a temperature of 500.degree. C. to 700.degree. C. is required to remove water from the annealing separator on the steel surface prior to high temperature annealing. PA0 (3) Such heating or soaking is, however, not always reliable for the complete removal of water, but it is sometimes likely that the remaining water may be released during high temperature annealing. This brings about the lack of uniformity in the composition of the annealing atmosphere, resulting in the lack of stability in secondary recrystallization and the production of steel not having good magnetic characteristics. PA0 (4) The water released during the high temperature annealing of steel sheets brings about an increase in the oxygen potential thereof, and thereby causes the excessive oxidation of the sheet surfaces, resulting in the production of sheets having inferior magnetic and mechanical properties.
The annealing separator is usually used in the form of a slurry obtained by dispersing it in water, and is applied to the steel sheet by spraying or roll squeezing after continuous decarbonization annealing. The annealing separator applied in the form of a slurry adheres closely to the steel sheet when it has been dried. The separator consisting mainly of magnesia has a high degree of solid-phase reactivity as hereinabove stated. This method of application, however, has a number of disadvantages, including the following:
These problems are due to the use of an aqueous suspension of the annealing separator, and can, therefore, be overcome if the annealing separator in dry powder form is applied directly to the surface of the steel sheets. Japanese Pat. Publications Nos. 12211/1964 and 11393/1982 disclose the electrostatic application of the annealing separator in dry powder form.
According to the method disclosed in Japanese Pat. Publication No. 12211/1964, the electrostatic application of the powder is effected by introducing it into the space between the electrode on which a positive corona discharge is formed and the surface of the steel sheet. It states that the annealing separator includes a wide range of substances, such as calcium oxide, alumina, silica or other heat-resistant oxides, lime and the like, and that though the invention is described by way of example with reference to the use of magnesia, such as MgO, it is obvious that the invention is not limited thereto. As regards the magnesia powder, it merely states that the grain size into which magnesia is divided is not critical, but is sufficient if it is fine enough to be carried by air, as hereinafter described. It is sufficient to use magnesia having a particle diameter which passes through a sieve having 325 meshes per inch, or which is about 44 microns. This method is difficult to employ successfully for practical application, since the formation of a positive corona discharge on the electrode brings about a poor charging efficiency resulting in poor adherence of the powder to the steel sheets. Moreover, the method does not enable the formation of a uniform glass-like film on the steel surface.
Japanese Pat. Publication No. 11393/1982 discloses a method which comprises applying a small quantity of a slurry consisting mainly of a magnesium oxide to form a good glass-like film as an undercoating on a silicon steel sheet, drying it, and charging particles of an annealing separator on the film to cause them to adhere to the surface of the sheet serving as an electrode. As regards the annealing separator for preventing sticking, it states that the method uses heavy magnesia, alumina, zirconium oxide, silicic acid, titanium oxide, nickel oxide, manganese oxide, calcium oxide, chromium oxide, molybdenum oxide or boron oxide, or a mixture or composite thereof. These oxides are used in the form of a powder having a particle size of 100 mesh (preferably 325 mesh). The method, however, lacks stability for continuous operation.
Although a variety of heat-resistant oxides in powder form are electrostatically applied as an annealing separator, the cohesion of the powder, resulting from absorbing moisture, causes the blocking of the apparatus used for the electrostatic application of the powder. This prevent a long period of reliable operation. A long period of reliable operation requires the use of a fully dried powder in a completely dry environment, but the complete removal of moisture from the powder is difficult to achieve on the spot in industrial production and requires expensive equipment.